1. Field of the Invention
The present invention relates to a communication system using CDMA (Code Division Multiple Access) and a control method of tracking the phases of spreading codes.
2. Description of the Related Art
CDMA is used for multiplexing a plurality of communication channels on the same frequency band by using a plurality of different spectrum spreading codes.
A base station spreads a transmitting signal to each terminal station by multiplying the transmitting signal by a spreading code specific to each communication channel, and transmits a multiplexed signal in the form of a plurality of mixed spread signals of communication channels by using a single carrier frequency. Each terminal station de-spreads a received signal by multiplying the reception signal by the code (de-spreading code) specific to its communication channel which is the same as the spreading code used by the base station, to thereby derive only a correlated signal of its channel. In this case, other channel signals are not correlated and become noise components because different spreading codes and de-spreading codes are used. The level of noise components can be lowered to a degree not to interfere with communications.
Attention has been paid to CDMA as a method of considerably improving the use efficiency of communication frequencies. CDMA has been adopted in the U.S.A. as a standard (IS-95) of digital cellular mobile communication systems and is now in practical use in this nation. The IS-95 standard uses orthogonal codes as spreading codes on a forward link for signal transmission from the base station to each terminal station.
Examples of orthogonal codes are shown in FIG. 11 being indicated at W0, W1, W2 and W3. An orthogonal code has the nature that a sum of the products of two arbitrary codes in a code group within an orthogonal unit becomes zero.
FIG. 10 shows a radio communication system including of a plurality of base stations 401 (401-1 to 401-j) interconnected by a wired network and a plurality of terminal stations 402 (402-1 to 402-n) distributed within a communication area of each base station. According to the IS-95 standard, the base station assigns specific orthogonal codes Wi (i=1 to n) to a plurality of terminal stations (or communication channels) within the communication area of the base station. The base station spreads a signal or data destined for a terminal station i with an orthogonal code Wi specific to the terminal station and transmits it. The terminal station de-spreads the received signal with the orthogonal code Wi specific to it so that signal components of all other channels are removed during the de-spreading process and they do not become interference signals.
Such communication systems, which use spectrum spreading with orthogonal codes for communications from a base station to terminal stations over forward links, are disclosed, for example, in U.S. Pat. No. 5,103,459.
In order to utilize the advantages of orthogonal codes, it is necessary to ensure perfect synchronization of orthogonal code timings between a subject reception channel and other reception channels during the de-spreading process. If there is any shift of timings of orthogonal codes on a plurality of multiplexed channels, orthogonality is lost so that received signal components of other reception channels become interference signals relative to the signal of the subject reception channel and its signal-to-noise (S/N) ratio is degraded.
Even if each terminal station spreads transmitting data by using orthogonal codes, the transmitting signal of each channel is received asynchronously at the base station because each terminal station transmits it independently over the reverse link. Therefore, the transmitting signals of respective channels become interference signals and a reception S/N ratio is degraded. Orthogonal codes without synchronization used as spreading codes as in the above case limit the number of connection channels. For example, in order to realize a reception S/N ratio of 10 dB, the number of connection channels via reverse links becomes about one tenth of the number of forward links.
For the above reason, a conventional CDMA communication system uses orthogonal codes only for one-to-N transmission links in order to easily synchronize timings of orthogonal codes between channels, i.e., only for forward links from a base station to terminal stations. For N-to-one transmission reverse links over which a plurality of terminal stations transmit signals independently, non-orthogonal codes such as pseudo-noise (PN) codes have been used for spreading transmission signals.
JP-A-7-254867 discloses a CDMA communication system in which a base station feeds a delay information on the channel back to a mobile terminal station, but does not refer to use of an orthogonal code therein.